As the prevalence of multimedia and window OS system, the mouse and track ball become essential devices for computer for pointing and sending command. Taking mouse as an example, the mouse is handled on a mouse pad with slight friction and the movement of mouse will cause a corresponding movement of cursor in screen. The corresponding movement of cursor by moving a mouse is achieved by an optical encoder. FIG. 12 shows the internal view of a mouse. A circuit board 81 is placed in front of the mouse body 80 and has three micro-switches corresponding to left, central right button on case of mouse.
The circuit board 81 has an optical encoder 90 on rear side thereof which comprises an X-axis encoder 91 and a Y-axis encoder 92. Each X/Y-axis encoder has a grating wheel 910/920 and an optoelectronic element 93/94. One end of the shaft of the grating wheel is in contact with the surface of the ball 95 such that the grating wheel 910, 920 will rotate in response to the rolling of ball 95.
As shown in FIG. 13, taking the Y-axis encoder as an example, the optical element 94 including a light source 941 and a light receiver 942 is arranged opposite to the slit of the grating wheel 920. The slit will shield intermittently the light from the light source 941 when the grating wheel rotates. The light receiving ends PT1 and PT2 of the light receiver 942 are functioned to determine the rotation direction of the grating wheel 920 by receiving the intermittent light.
As shown in FIG. 14, the relative movement of the grating wheel 920, the light source 941 and the light receiver 942. in FIG. 14A, the two light receiving ends PT1, PT2 of the light receiver 942 is blocked by the grating wheel the digital counter part is shown in T1 of FIG. 15 wherein PT1 and PT2 are (0,0). As the grating wheel 920 rotating, the PT2 receives light from slit 920A while the PT1 is still blocked, therefore, in period T2, (PT1,PT2) is (0,1). As the grating wheel 920 still rotating, both PT1 and PT2 receive light from slit 920A, as shown in FIG. 14C. Therefore, in period T3 of FIG. 15, (PT1,PT2) is (1,1). As the grating wheel 920 still rotating, PT1 receives light from slit 920A, while PT2 is blocked by slit 920A, as shown in FIG. 14D. Therefore, in period T4 of FIG. 15, (PT1,PT2) is (1,0).
However, the working waveform shown in FIG. 15 will be influenced by the light interference. This phenomenon can be more clearly explained with reference to FIG. 16 where D denotes light source, C denotes the grating wheel in front of the light source D, C1 and C2 denote the slits on grating wheel C, and S denotes the sensor opposite to light source D and moving up and down along T-axis.
FIG. 17 shows the wave distribution of light passing through slits C1 and C2, wherein N is the intensity and H is the field distribution.
When slit C2 is blocked, the waveform detected by sensor S is N1=(H1).sup.2.
When slit C1 is blocked, the waveform detected by sensor S in N2=(H1).sup.2.
If the light from slit C1 and C2 do not interfere with each other, the waveform detected by sensor S is N12=N1+N2, as shown in FIG. 18.
However, due to the interference, the really waveform is N12-N1+N2, as shown in FIG. 19.
The interference of light will influence the resolution of PT1 and PT2. Again taking a glance at FIG. 14A, the detection signal on PT1 and PT2 is desirable to be (0,0) when PT1 and PT2 are blocked by the opaque part between slits. However, the PT1 and PT2 may be erroneously detected to receive light caused by the light interference.
There are two methods to prevent the problem caused by light interference.
1. Minimize the distance between the light receiver and the grating wheel. However, the manufacturing technique is different. PA1 2. Increase the size of the slit to be three times of light receiving end of the light receiver. However; the resolution is degraded.
As mentioned above, the prevention of light interference is at the expense of ost and resolution, there is still much to be improved.